Methane clathrate (also called methane hydrate, hydromethane, methane ice or “fire ice”) comprises a solid clathrate compound (more specifically, a clathrate hydrate) in which a large amount of methane is trapped within a crystal structure of water, forming a solid similar to ice. Within methane clathrate deposits, the small non-polar molecules (typically gases) are trapped inside “cages” of hydrogen-bonded water molecules. In other words, clathrate hydrates are clathrate compounds in which the host molecule is water and the guest molecule is typically a gas. Since the trapped molecules do not bond to the lattice, the clathrate hydrates are not chemical compounds, and the formation and decomposition of clathrate hydrates are first-order phase transitions and not chemical reactions.
Methane clathrates occur at under water depths of less than 2000 meters, for example adjacent to polar continental sedimentary rocks where surface temperatures are less than 0° C. and in oceanic sediment at water depths greater than 300 m where the water temperature is around 2° C. In addition, deep lakes may host gas hydrates as well. Continental deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less than 800 m depth. Further, oceanic deposits seem to be widespread in the continental shelf and can occur within the sediments at depth or close to the sediment-water interface. Additionally, methane clathrate deposits may cap even larger deposits of gaseous methane.
Methane hydrates sometimes form from methane gas released as byproduct of deep sea drilling or from release of methane gas along oceanic geological faults. In some regions (e.g., the Gulf of Mexico), methane in clathrates may be at least partially derived from thermal degradation of organic matter. When released, the methane gas floats upward toward the surface of the water. In warm waters, the methane gas may be released into the atmosphere. In colder climates and at deep sea levels or in deep lakes, at least a portion of the methane gas crystallizes on contact with cold water. The crystallized methane gas flows with deep water currents, eventually settling in deposits. Such deposits often exist in the ocean near the continental shelves.
The worldwide amounts of methane bound in clathrate hydrates is conservatively estimated to total twice the amount of methane to be found in all known fossil fuels on Earth. Testing of such deposits indicates that the average methane clathrate hydrate composition includes one mole of methane for every 5.75 moles of water. The average observed density has been around 0.9 grams per cubic centimeter. Based on these averages, a typical liter of methane clathrate solid would contain approximately 168 liters of methane gas (at STP).